Photogravure or gravure printing is a generally known printing technique whereby intaglio engravings of an image to be printed on a substrate (usually paper) are formed on the surface of a gravure cylinder. Intaglio engravings are those where the elements to be printed are formed below the surface of the gravure cylinder, typically by engraving into a metallic cylinder, thereby forming ink-retaining groves or cells in the cylinder. Used in conjunction with the etched or engraved gravure cylinder is the very important doctor blade which controls the amount of ink transferred from the printing surface of the cylinder to the paper or other substrate material.
Another method of printing is flexographic printing. This is also a technique where ink is carefully wiped from the surface of an engraved cylinder before the ink in the cells is transferred. Here, too, the doctor blade is very important.
A doctor blade is usually a long, thin resilient strip of metal that is mounted in the printing machine parallel to the axis of rotation of the cylinder, and the edge of the doctor blade comes into contact with the surface of the cylinder. When the machine is in operation, the doctor blade controls the amount of ink transferred to the substrate by wiping the excess ink from the surface of the cylinder, thereby leaving only the ink within the etching or engravings for transfer.
While proper etching or engraving of the cylinder is extremely important to the quality of the ultimate printed product, the significance of the doctor blade cannot be discredited. Indeed, even if the cylinder has been properly engraved, improper doctor blade design and usage will prevent the obtaining of a proper printed image.
Doctor blades for many years have been a two-part configuration of a backup blade and a doctor blade. Both the backup blade and doctor blade are held in a blade holder so that the doctor blade urges against the cylinder. Even though the doctor blade will wear as a result of the wiping action against the cylinder and must eventually be replaced, the backup blade is usually reusable. The backup blade adds support to the very thin doctor blade and securely holds the thin doctor blade in the blade holder. This multi-blade construction has many inherent drawbacks, not the least of which are accurately aligning and securing the blades in the blade holder, which requires manual skill and extended downtime for the press, and ink buildup between the blades which must be routinely eliminated.
A more recent development in doctor blade configurations involves the creation of doctor blades consisting of several layers of dissimilar material bonded or laminated together in some fashion to form a multi-layer doctor blade. As shown in U.S. Pat. No. 4,895,071 to Benton, the two-part doctor blade disclosed therein is comprised of a backup section and a doctoring section which are joined together. The doctoring section is that portion of the blade that actually contacts the gravure cylinder and is made from doctor blade steel. The usually wider backup portion; however, is made of less costly tin-free steel (TFS) or TFS coated with chromium for corrosion protection. Since the TFS is less expensive than the doctor blade steel, an economy in manufacture is perceived.
Although the manufacture and use of such multi-layer doctor blades is now known, there are significant disadvantages that need to be overcome. The most obvious drawback to the multi-layer construction is the need to accurately and effectively bond together two dissimilar materials to form the blade. The bonding technique is important just to achieve the bond, and the selection and sizing of the material is important from the standpoint of achieving proper rigidity of the composite blade. Even after the composite blade is formed difficulties persist. During use in the printing process, the various layers of the blade can separate, even minutely, and create spaces where ink and other contaminates from the cylinder can collect, dry and affect print quality. As mentioned above, bonding or laminating of the two dissimilar materials greatly affects the rigidity of the resultant blade. As a result, during the printing process, the pressure exerted by the doctor blade against the cylinder oftentimes must be constantly increased in order to insure proper wiping; however, the increased pressure causes increased friction against the cylinder and causes both the doctor blade and the cylinder to wear prematurely. As a concomitant effect, the fact that these multi-layer doctor blades are subject to the above disadvantages (which require frequent replacement of the blade), there is a resultant downtime of the press during the replacement period of either the cylinder or the blade which is a very significant economic consideration.
In light of these deficiencies of the prior doctor blade configurations, a more durable and cost efficient doctor blade is called for.